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Am I missing anything?

Just trying to make sure I've covered everything. I've been called out on a 1987 Trane 40 ton (2 20 ton circuts) air cooled chiller. The screw compressor was changed out in 1997 on Circuit 2.
Circuit 2 will run with identical pressures to Circuit 1 for 2-3 minutes, then the suction pressure will bottom out and the circit locks out on low pressure. This happens so fast, I had to viedo tape my gauges to see the drop before lock out. On advise from a Trane Rep, I changed the txv. At that time I also changed liquid dryer and suction dryer core. No change at all. Suspecting a restriction in the barrel, I isolated the liquid line just before the new dryer, closed the king valve on the suction at the compressor, and charged 250 psi of Nitrogen at the new liquid line dryer. This resulted in 20 psi at the inlet of the suction filter.
Showing this to the facilities manager resulted in 2 hour discussion on refrigeration theory, and him very skeptical that the barrel is restricted. With him watching, I dumped the Ntg charge, isolated the compressor at the king valves, charged 250 through the liquid line, then showed him the gauge, reading 20 psi at the suction king valve. Still don't believe me!
Does anyone out there know of anything that I'm missing. At this point I've told him we'll be happy to quote a new chiller. Thanks!

Just trying to make sure I've covered everything. I've been called out on a 1987 Trane 40 ton (2 20 ton circuts) air cooled chiller. The screw compressor was changed out in 1997 on Circuit 2....Does anyone out there know of anything that I'm missing....

you are missing a model number. i don't believe that trane made (or currently produces) a 20 ton screw compressor.

Originally Posted by coolinginga

...Suspecting a restriction in the barrel, I isolated the liquid line just before the new dryer, closed the king valve on the suction at the compressor, and charged 250 psi of Nitrogen at the new liquid line dryer. This resulted in 20 psi at the inlet of the suction filter....

if you have electronic expansion valves, they act like a solenoid when closed...so I can believe that you only got 20 psig at the suction line.

Originally Posted by coolinginga

...At this point I've told him we'll be happy to quote a new chiller. Thanks!

if you really do have such a restriction, then it should be relatively easy to find. i think that a new chiller is a bit premature until you 'see' the problem.

Just trying to make sure I've covered everything. I've been called out on a 1987 Trane 40 ton (2 20 ton circuts) air cooled chiller. The screw compressor was changed out in 1997 on Circuit 2.

Trane didn't make screw chillers in 1987 and they never made a screw chiller smaller than 70 tons. You need to post the model and serial numbers for this chiller.

Originally Posted by coolinginga

Circuit 2 will run with identical pressures to Circuit 1 for 2-3 minutes, then the suction pressure will bottom out and the circit locks out on low pressure. This happens so fast, I had to viedo tape my gauges to see the drop before lock out. On advise from a Trane Rep, I changed the txv. At that time I also changed liquid dryer and suction dryer core. No change at all.

You need to try to get superheat and subcooling if possible. If this is a recip or scroll chiller as I suspect, you need to check the solonoid valve.

Originally Posted by coolinginga

Suspecting a restriction in the barrel, I isolated the liquid line just before the new dryer, closed the king valve on the suction at the compressor, and charged 250 psi of Nitrogen at the new liquid line dryer. This resulted in 20 psi at the inlet of the suction filter.

Your also charging your nitrogen across your TXV and possibly your liquid line solonoid valve which will cause a HUGE pressure drop.

Thanks for the responses. I'll post the model and serial numbers tomorrow, I'll have to go by our office to get them. When I replaced the txv I also replaced the solenoid due to some repiping that had to be done. (it was easier to replace it than to try to reuse the old one.) I can't get superheat or subcooling because after changing the txv, the chiller blew out a weld joint in the discharge piping, (not related to anything I was working on), and is sitting with a nitrogen charge since. The coustomer doesn't want to invest in 80#of R22 until they know the chiller is going to run. The solenoid has a manual lift stem, and it's open, and the expansion valve is a thermal expansion valve. I have held the bulb in my hand, assuming that would be enough heat to open the valve. Btw, both circuits share a common barrel, and circut 1 is operating fine.

Hi guys, bklyntek, sounds like garbage in the system,
Moisture causing an ice ball at txv, resulting in loss of flow.
I would log working ckt,/ all press-temp & approaches,
Spht & subcooling. Then 3x evacuate to 500 microns.
Use all you readings to help diagnose.